Advances in the immunology have identified that interleukin (IL)‑17 and IL‑35 are cytokines with diverse functions, serving important roles in autoimmune diseases and chronic inflammation. Checkpoint inhibitor pneumonitis (CIP) is focal or diffuse lung inflammation induced by immune checkpoint inhibitors and the underlying pathogenesis has not been fully explored. The aim of the present study was to investigate the roles of IL‑17A and IL‑35, and the correlation between their levels and different T cell subsets in CIP. The levels of IL‑17A and IL‑35 in peripheral blood and bronchoalveolar lavage fluid (BALF) were measured in patients with non‑small cell lung cancer (NSCLC) with CIP, and the corresponding controls. The percentages of helper T lymphocyte (Th) 1, Th2 and Th17 cells, and regulatory T cells (Tregs) in the peripheral blood were synchronically detected. Serum levels of IL‑17A and IL‑35 were significantly increased at the time of CIP diagnosis compared with the baseline, and significantly decreased upon clinical recovery or improvement. IL‑17A and IL‑35 were also increased in the BALF during the development of CIP compared with the baseline. Serum levels of IL‑17A were positively correlated with the percentages of Th1 and Th17 cells as well as the ratio of Th17 to Tregs, but negatively associated with the frequency of Tregs in CIP. Serum levels of IL‑35 were positively correlated with the percentages of Th1 and Tregs, and with the ratio of Th1 to Th2 cells in CIP. A higher frequency of Th1 and Th17 cells, as well as higher ratios of Th17 to Tregs and Th1 to Th2 cells were detected upon development of CIP comparing with the baseline. These data suggested that the activation of Th1 and Th17 cells, as well as Treg inhibition contributed to the imbalanced ratios of Th1 to Th2 and Th17 to Tregs, which resulted in increased secretion of IL‑17A and IL‑35 in the plasma and BALF; this may present a valuable index to monitor the development and severity of CIP in patients with NSCLC receiving immunotherapy.